This invention relates to a method of making a digitally disruptable weakening line in sheet metal, the line preferably to include a fractured but integral section. The invention relates more especially to a method of making an easy-opening can end closure defined at least in part by such a line.
A prior method of making can end closures having a disruptible weakening line characterized by an integral but fractured section is disclosed in U.S. Pat. No. 3,881,437. The patented method contemplates, essentially, three successive steps which may be briefly stated to involve (a) bending or forming sheet metal to provide a closure periphery having a wall arcuate in transverse section, (b) shear-coining the wall longitudinally to create an integral but fractured section, and then (c) swaging the metal to seal or tighten the edges at the fractured section. This approach is considered to have considerable merit over the numerous and usually more complex methods previously known for making other types of weakening lines for comparable purposes. It was also recognized, as indicated in the copending application above cited that, especially when closures were to be provided in tougher metal, advantages could be attained over the patented method if steps (a) and (b) were effected substantially simultaneously rather than in sequence.